HIV infection leads to CD4 lymphopenia and immunosuppression, which can be successfully improved with antiretroviral therapy (ART) in the majority of people. Approximately 20 percent of HIV patients starting ART especially with lower CD4 counts may develop an aberrant immune response known as immune reconstitution syndrome (IRIS). IRIS encompasses the paradoxical worsening of the manifestations of underlying opportunistic diseases (paradoxical IRIS) or the abrupt presentation of previously occult opportunistic disease (unmasking IRIS) in patients who recently started ART. Symptoms of IRIS can range from uncomplicated localized reactions to severe and systemic manifestations and, more rarely, autoimmune phenomena. Clinical studies have identified two main risk factors for IRIS: severe CD4 lymphopenia and the presence of opportunistic infections even if clinically silent (M. tuberculosis, M. avium complex, C. neoformans or other fungi, or viral pathogens) prior to ART initiation. The pathogenesis of IRIS in HIV infection remains unclear and there is no animal model that adequately mimics the clinical observations. In order to study the clinical predictors, biomarkers and pathogenesis of IRIS we are conducting a prospective observational clinical trial of ART-naive HIV+ patients with severe CD4 lymphopenia (<100 cells/L) who are followed after initiation of ART. We previously reported that T cells from IRIS patients bore a highly activated phenotype (effector cells with high PD-1 expression) showing evidence of profound antigenic stimulation and had a slower recovery of naive CD4 T cells following the IRIS events suggesting a significant antigenic drive of T cell activation and expansion that becomes dysregulated as severe immunosuppression reverses. In continuation of that work, we showed that the activated CD4 T cells predominantly recognize the underlying opportunistic pathogen and mount polyfunctional cytokine responses while the T cell responses to other pathogens, including HIV itself, remain intact. More recently, we showed that monocytes play a prominent role in TB-IRIS and may serve as important predictors as well as potential therapeutic targets. Finally, comparing HIV persons and HIV-seronegative with the same pathogen (MAC) we found that the CD4 responses against MAC during IRIS are far greater than those of HIV-seronegative patients with MAC. In addition, the MAC-specific CD4 cells have unique features expressing high levels of EOMES, low Tbet and have evidence of cytotoxic potential. We also studied patients with fluorodeoxyglucose (FDG)-PET imaging and found that higher glucose uptake (total glycolytic activity and total glycolytic volume) and higher standardized max update pre-ART were associated with IRIS incidence after therapy initiation. PET measurements correlated with plasma inflammatory cytokines. In addition, higher Glut-1 expression (main glucose transporter) were observed on CD4 T cells and monocytes of patients who developed IRIS. These observations highlight a seminal role of metabolism in general and glycolysis more specifically in IRIS events which may help identify new therapeutic targets. Importantly, we described that similar IRIS phenomena can occur in people without HIV infection, specifically some marrow recipients with primary immunodeficiencies who had disseminated mycobacterial infections and recovered their pathogen-specific immune responses. Previous studies have shown that inflammatory mediators such as IL-6 (and CRP), D-dimer, IL-8, TNF, IL-17, IL-1 and IFNg are elevated in IRIS events and likely are primarily responsible for the symptomatology of the syndrome. In a small case series we showed that TNF blockade may be a useful means to treat refractory to corticosteroids IRIS. Following up on this observation we conducted a phase I pilot study of an oral phosphodiesterase 4 inhibitor (CC-11050) in people living with HIV who had suppressed plasma viremia to assess tolerability and potential interactions with HIV therapy. We found the drug was well tolerated and also lowered slightly the plasma IL-8 levels. Despite the significant improvement of morbidity and mortality in HIV infection in the ART era, mortality in HIV+ patients is still in excess of what is expected based on age and strongly relates to the degree of immunodeficiency as measured by CD4 T cell counts and to the degree of residual inflammation measured by IL-6, CRP, sCD14 and D-dimer. These markers appear to be linked more to activation of the innate system than the T cells including an activated monocyte phenotype that was shown to be an independent predictor of progression of atherosclerosis in treated HIV patients and are related to mortality and cardiovascular events. We had previously evaluated these critical biomarkers at a cohort of HIV infected persons diagnosed and treated at the earliest possible stages of HIV infection and found that despite the early treatment, inflammatory markers did not completely normalize after two years of ART although D-dimer levels did normalize. In another study of untreated and treated chronically infected HIV patients and found that tissue factor expression on monocytes, a protein that triggers activation of the extrinsic pathway of the coagulation cascade, is heightened in HIV despite ART and is linked to both coagulation and increased inflammatory cytokines. The pathogenic role of tissue factor was further confirmed in an SIV model. Treatment with Ixolaris (an inhibitor of tissue factor) blocked activation of coagulation in vitro with human and non-human primate cells. We also found that levels of IL-6 were independently associated with atherosclerosis and mortality in HIV infected persons with suppressed plasma viremia and plasma levels of soluble tissue factor were associated with progression of carotid intima media thickness (cIMT). Immune-based therapies and anti-inflammatory therapies are thus still being pursued with the objective to improve immune restoration and function in CD4 lymphopenic states. Newer strategies for decreasing residual inflammation in IRIS and in chronically treated patients are being pursued including commonly used medications such as statins and aspirin.These observations are being followed by further evaluation of the coagulopathy in HIV and potential interventions to improve it. Finally in our Idiopathic CD4 lymphopenia work, we developed a humanized mouse model to study the behavior of both peripheral blood mononuclear cells and also mobilized hematopoietic stem cells from ICL patients and controls. Using this model we were able to show that there is heterogeneity in ICL patients with approximately half being able to reconstitute the empty mouse host just as healthy controls. The ones who could not showed evidence of either depressed proliferative capacity of PBMC or increase death. In contrast hematopoietic cells were able to develop normally into T cells. This mouse model provides us with a tool to classify and further study ICL patients evaluating potential defects of T cell development or peripheral expansion and survival.